Electronic circuit with a capacitive sensor for measuring a physical parameter and method of activating the electronic circuit

Abstract

The electronic circuit has a capacitive sensor that includes two capacitors mounted in differential, whose common electrode can move relative to each fixed electrode of the two capacitors to alter the capacitive value of each capacitor. The electronic circuit has an interface connected to the capacitive sensor, which includes a charge transfer amplifier unit connected to the common electrode, an integrator unit, and an excitation unit arranged between the output of the first integrator unit and the sensor to polarise each fixed electrode of the sensor capacitors at a determined voltage value. A compensation capacitor is connected to the input of the integrator unit. The electronic circuit interface includes comparison means for comparing the output voltage with a comparison voltage to control disconnection of the compensation capacitor at the integrator unit input, if the deviation between the output voltage and the comparison voltage exceeds a determined voltage threshold.

What is claimed is: 
     
       1. An electronic circuit comprising:
 (a) a capacitive sensor disposed to measure a physical parameter, wherein the sensor includes at least two differential-mounted capacitors, wherein each differential-mounted capacitor has a fixed electrode, wherein the at least two differential-mounted capacitors share a common electrode that is movable relative to each fixed electrode of the at least two capacitors, and wherein the common electrode is moved to alter the capacitive value of each capacitor during measurement of the physical parameter; and 
 (b) an interface connected to the capacitive sensor, wherein the interface includes
 (i) a charge transfer amplifier unit connected to the common electrode, 
 (ii) an integrator unit disposed to integrate the charges supplied by the charge transfer amplifier unit and to supply an output voltage, wherein the integrator unit includes a first compensation capacitor at input, 
 (iii) an excitation unit arranged between the output of the integrator unit and the sensor, wherein the excitation unit is disposed to polarize each fixed electrode of the sensor capacitors at a determined voltage value, and 
 (iv) comparison means disposed to compare the output voltage with a comparison voltage and to control disconnection of the compensation capacitor at the integrator unit input if the deviation between the output voltage and the comparison voltage is above a determined voltage threshold; 
 
 wherein the comparison means is a dynamic comparator, wherein the comparison means includes at input a threshold capacitor that is connected to a control capacitor and disposed to define a capacitive ration, and wherein the capacitive ratio determines, by multiplying by a voltage value that corresponds to a low potential subtracted from a high potential of the supply voltage source, the determined voltage threshold; 
 wherein a first electrode of the threshold capacitor is connected at input via a first switch of the comparator to the high potential and via a second switch to the low potential of the supply voltage source, wherein a second electrode of the threshold capacitor is connected to a second electrode of the control capacitor and defines a connection node, wherein the first electrode of the control capacitor is connected, via a third switch, to the comparison voltage and, via a fourth switch, to the output voltage of the first integrator unit; and 
 wherein the switches are controlled by the dynamic comparator, and wherein the comparator compares the output voltage subtracted from the comparison voltage, and the comparison voltage subtracted from the output voltage. 
 
     
     
       2. The electronic circuit according to  claim 1 , wherein the electronic circuit interface further comprises:
 (v) a second integrator unit with the same structure as the first integrator unit, disposed to integrate the charges supplied by the charge transfer amplifier, wherein the second integrator unit includes at input a second compensation capacitor that can be disconnected like the first compensation capacitor via the comparison means if the deviation between the output voltage of the first integrator unit and the output voltage of the second integrator unit is above a determined voltage threshold, wherein the output voltage of the second integrator unit defines the comparison voltage, and wherein the first and second integrator units are arranged to operate alternately and cyclically in each measuring cycle; and 
 (vi) a second excitation unit arranged between the output of the second integrator unit and the sensor, wherein the second excitation unit is disposed to polarize each fixed electrode of the capacitors of the sensor at a determined voltage value that is inverse of the voltage value controlled by the first excitation unit, wherein the first and second excitation unit are arranged to operate alternately and cyclically in each measuring cycle, each with their respective integrator unit, so that the electronic circuit operates entirely symmetrically to measure the physical parameter. 
 
     
     
       3. The electronic circuit according to  claim 2 , wherein each integrator unit comprises:
 (1) an operational amplifier; 
 (2) an integration capacitor connected between an inverter input of the amplifier and the amplifier output; 
 (3) a first switch connected between the output of the charge transfer amplifier unit and the inverter input of the amplifier; and 
 (4) a second switch connected between the output of the charge transfer amplifier unit and the non-inverter input of the amplifier, wherein the non-inverter input of the amplifier is polarised at a reference voltage, wherein each switch of each integrator unit is controlled and alternately closed in each normal measuring cycle of the electronic circuit; 
 wherein the first and second compensation capacitors are connected by a first electrode at the input of the first and second switches of each integrator unit; 
 wherein a second electrode of each compensation capacitor is each connected to the output of the respective integrator unit by a third switch, and connected to the reference voltage by a fourth switch, wherein the third and fourth switches of each integrator unit are controlled by the control means; and 
 wherein the control means is disposed to open the third and fourth switches, and thus disconnect the compensation capacitors, if the voltage deviation between the output voltage of the first integrator unit and the output voltage of the second integrator unit is above the determined voltage threshold. 
 
     
     
       4. The electronic circuit according to  claim 1 , wherein the comparison means is a dynamic comparator. 
     
     
       5. The electronic circuit according to  claim 1 , wherein the connection node of the threshold and control capacitors is connected to a gate of a first NMOS transistor and to the drain thereof by a fifth switch, the source of the first NMOS transistor being connected to the low potential terminal, wherein the drain of the first NMOS transistor is connected to the drain of a first PMOS transistor, wherein a source of the first PMOS transistor is connected to the high potential terminal, the gate of the first PMOS transistor being polarised by the polarising voltage, wherein the polarising voltage also polarises the gate of a second PMOS transistor, wherein a source of the second PMOS transistor is connected to the high potential terminal, the drain of the second PMOS transistor being connected to the drain of a second NMOS transistor, where a source of the second NMOS transistor is connected to the low potential terminal, and the gate of the second NMOS transistor is connected to the drains of the first NMOS and PMOS transistors, and wherein the drains of the second NMOS and PMOS transistors are connected to an input of a first D flip-flop clocked by a first clock signal and to an input of a second D flip-flop clocked by a second clock signal, the output of each flip-flop being connected to a NOR logic gate, and wherein the output of the NOR logic gate supplies the comparator control signal. 
     
     
       6. A method of activating the electronic circuit according to  claim 1 , for controlling the level of an output voltage of at least one integrator unit, for measuring a physical parameter, the method including at least two phases for each successive measuring cycle, the method comprising the steps of:
 (a) polarising, in a first phase, each fixed electrode of the capacitors of the sensor at the output voltage of the integrator unit via the excitation unit; and 
 (b) polarising, in a second phase, via the excitation unit, the fixed electrode of the first capacitor at a high voltage or at a low voltage of a supply voltage source of the electronic circuit, and in polarising the fixed electrode of the second capacitor inversely to the fixed electrode of the first capacitor, at a low voltage or at a high voltage of the voltage source; 
 wherein during successive physical parameter measuring cycles, the activated comparison means compares the output voltage and a comparison voltage and supplies a control signal to disconnect the compensation capacitor if the deviation between the output voltage and the comparison voltage is above a determined voltage threshold, and wherein the compensation capacitor remains disconnected in the subsequent measuring cycles while the voltage deviation remains higher than the voltage threshold. 
 
     
     
       7. The method according to  claim 6 , wherein the comparison performed by the comparison means to check whether the deviation between the output voltage and the comparison voltage is higher than the determined voltage threshold lasts for three successive measuring cycles, and wherein the comparison in the comparison means is continually repeated while the voltage deviation remains greater than the voltage threshold. 
     
     
       8. The method according to  claim 7 , wherein, to perform a comparison cycle, the comparison means performs a first comparison by subtracting the output voltage from the comparison voltage, and a second comparison by subtracting the comparison voltage from the output voltage, and wherein, if the result of at least one of the subtractions is higher than the determined voltage threshold, the control signal supplied by the comparison means disconnects the compensation capacitor. 
     
     
       9. The method according to  claim 6 , wherein the electronic circuit includes a second integrator unit that has a second compensation capacitor and a second excitation unit, wherein the second compensation capacitor and the second excitation unit have the same structure as the first integrator unit and the first excitation unit, but operate alternately to measure a physical parameter, the method including four phases for each successive physical parameter measuring cycle, wherein, after the first and second phases, the method further comprising the steps of:
 (c) polarising, in a third phase, via the second excitation unit, each fixed electrode of the sensor capacitors at the output voltage of the second integrator unit; and 
 (d) polarising, in a fourth phase, via the second excitation unit, each fixed electrode of the sensor capacitors, with one fixed electrode polarised at the high voltage and the other fixed electrode at the low voltage, inversely to the polarisation of the electrodes in the second phase; 
 wherein during the physical parameter measuring cycles, the comparison means performs a comparison between the first integrator unit output voltage and the second integrator unit output voltage, as the comparison voltage, for a duration that corresponds to three successive measuring cycles, and wherein, in each comparison cycle, the comparison means performs a first comparison of the first integrator unit output voltage subtracted from the second integrator unit output voltage, and a second comparison of the second integrator unit output voltage subtracted from the first integrator unit output voltage, and if the result of at least one of the subtractions is greater than the determined voltage threshold, a control signal supplied by the comparison means disconnects the two compensation capacitors. 
 
     
     
       10. An electronic circuit comprising:
 (a) a capacitive sensor disposed to measure a physical parameter, wherein the sensor includes at least two differential-mounted capacitors, wherein each differential-mounted capacitor has a fixed electrode, wherein the at least two differential-mounted capacitors share a common electrode that is movable relative to each fixed electrode of the at least two capacitors, and wherein the common electrode is moved to alter the capacitive value of each capacitor during measurement of the physical parameter; and 
 (b) an interface connected to the capacitive sensor, wherein the interface includes
 (i) a charge transfer amplifier unit connected to the common electrode, 
 (ii) an integrator unit disposed to integrate the charges supplied by the charge transfer amplifier unit and to supply an output voltage, wherein the integrator unit has
 (1) a first compensation capacitor at input, 
 (2) an operational amplifier with an inverter input, a non-inverter input and an output, 
 (3) an integration capacitor, wherein the integration capacitor is connected between the inverter input and the output of the operational amplifier, 
 (4) a first switch connected between an output of the charge transfer amplifier unit and the inverter input of the operational amplifier, 
 (5) a second switch connected between the output of the charge transfer amplifier unit and the non-inverter input of the operational amplifier, wherein the non-inverter input of the operational amplifier is polarised at a reference voltage, wherein each switch of the integrator unit is controlled and alternately closed in each normal measuring cycle of the electronic circuit, and 
 
 (iii) an excitation unit arranged between the output of the integrator unit and the sensor, wherein the excitation unit is disposed to polarize each fixed electrode of the sensor capacitors at a determined voltage value, and 
 (iv) comparison means disposed to compare the output voltage with a comparison voltage and to control disconnection of the compensation capacitor at the integrator unit input, if the deviation between the output voltage and the comparison voltage is above a determined voltage threshold; 
 
 wherein the compensation capacitor is connected by a first electrode at an input of the first and second switches of the integrator unit; and 
 wherein the compensation capacitor has a second electrode that is connected to an output of the integrator unit by a third switch and to the reference voltage by a fourth switch, wherein the third and fourth switches of the integrator unit are controlled by a control means, and wherein the control means opens the third and fourth switches, thereby disconnecting the compensation capacitor, if the voltage deviation between an output voltage of the integrator unit and a comparison voltage is above the determined voltage threshold. 
 
     
     
       11. An electronic circuit comprising: (a) a capacitive sensor disposed to measure a physical parameter, wherein the sensor includes at least two differential-mounted capacitors, wherein each differential-mounted capacitor has a fixed electrode, wherein the at least two differential-mounted capacitors share a common electrode that is movable relative to each fixed electrode of the at least two capacitors, and wherein the common electrode is moved to alter the capacitive value of each capacitor during measurement of the physical parameter; and (b) an interface connected to the capacitive sensor, wherein the interface includes
 (i) a charge transfer amplifier unit connected to the common electrode, 
 (ii) an integrator unit disposed to integrate the charges supplied by the charge transfer amplifier unit and to supply an output voltage, wherein the integrator unit includes a first compensation capacitor at input, and 
 (iii) an excitation unit arranged between the output of the integrator unit and the sensor, wherein the excitation unit is disposed to polarize each fixed electrode of the sensor capacitors at a determined voltage value, and 
 (iv) comparison means disposed to compare the output voltage with a comparison voltage and to control disconnection of the compensation capacitor at the integrator unit input if the deviation between the output voltage and the comparison voltage is above a determined voltage threshold; 
 wherein each integrator unit comprises: 
 (1) an operational amplifier; 
 (2) an integration capacitor connected between an inverter input of the amplifier and the amplifier output; 
 (3) a first switch connected between the output of the charge transfer amplifier unit and the inverter input of the amplifier; and 
 (4) a second switch connected between the output of the charge transfer amplifier unit and the non-inverter input of the amplifier, wherein the non-inverter input of the amplifier is polarised at a reference voltage, wherein each switch of each integrator unit is controlled and alternately closed in each normal measuring cycle of the electronic circuit; 
 wherein the first and second compensation capacitors are connected by a first electrode at the input of the first and second switches of each integrator unit; wherein a second electrode of each compensation capacitor is connected to the output of the respective integrator unit by a third switch, and connected to the reference voltage by a fourth switch, wherein the third and fourth switches of each integrator unit are controlled by the control means; 
 and wherein the control means is disposed to open the third and fourth switches, and thus disconnect the compensation capacitors, if the voltage deviation between the output voltage of the first integrator unit and the output voltage of the second integrator unit is above the determined voltage threshold.